As the synthetic resin preforms that have been molded into the primary molded intermediates and are to be biaxially stretched and blow molded into bottles, there are known those preforms of a cylindrical shape having a closed end, which have been molded by an injection molding means or a blow molding means. As compared to the injection molded preforms, the blow molded preforms can be obtained at less expensive costs, more freely molded into various shapes, and are easy to adopt the laminated wall structures.
Thus, the blow molded preforms have the following advantages. Firstly, their cost of equipment is lower than the corresponding cost for injection-molded preforms, and the blow-molded products can be sold at a lower price than the injection molded ones. Secondly, because they are highly versatile in selecting shapes, the preforms can be molded into a shape that fits in with the appearance of a bottle to be molded. Thirdly, because they are easy to adopt the laminated wall structures, the thin-wall bottles can be obtained while desired physical properties can be effectively maintained.
However, the above prior art has a problem in that local thick areas tend to occur in the shoulder portion and on the underside of the bottom of the molded bottle. Because of these thick areas, the molded bottles have a bias of thickness in the walls, thus resulting in decreased precision in the overall wall thickness.